1. Technical Field
The present invention relates to a functional thin-film element usable in an organic EL element (organic electroluminescence element), inorganic EL element, solar cell, light modulation element, transistor element (FET element), and the like, a producing method of a functional thin-film element, and an article using a functional thin-film element.
2. Relevant Art
Development of functional elements is being accelerated by virtue of the recent progress of IT (information technology). Examples of functional elements include: an electroluminescence element configured to emit light; a solar cell configured to absorb light and convert it into energy; a light modulation element (based on liquid crystal, electrochromic, and the like) having a light transmittance to be modulated by turning on and off a voltage across the light modulation element; and a transistor element (FET element). Recently, there have been extensively conducted researches on functional elements for televisions (TV) such as a plasma display having a merit of higher brightness and wider viewing angle, and an organic electroluminescence display, and there have been frequently used functional elements in a flat panel display for vehicular navigation systems, a cellular phone, and an electronic paper, in the fields other than TV's.
In such functional elements represented by solar cells and light modulation elements, there is/are interposed a functional thin film or films between an anode and a cathode, in a manner to actively utilize movement of charge carriers (electrons and positive holes) in interfaces between both electrodes and functional thin film, or in an interface between the functional thin films, i.e., in the junction interface(s), thereby exhibiting electronic and optical functions.
Further explanation will be concretely provided, taking a recently attention-getting organic electroluminescence element, for example. Japanese Patent Application Laid-Open Publication No. 11-251066 and The Organic EL Element Technique, Technotimes Publication Co., Ltd. (Page 17) disclose an organic EL element including a transparent substrate (glass, resin), an anode (transparent electrode: ITO (indium tin oxide)) formed thereon, and a luminescent layer (functional thin film) and a cathode (Mg/Ag) formed on the anode.
In an organic EL element, application of a voltage between an anode and a cathode causes positive holes from the anode side and electrons from the cathode side to be injected into a luminescent layer (functional thin film) by overcoming potential barriers Δφ at junction interfaces between the anode and cathode and the luminescent layer, respectively. Injected positive holes and electrons are recombined to emit light which is then emitted from the anode (transparent electrode) side. Supposing here that ITO as the anode (transparent electrode) has an ionization potential φ2 of about −4.5 eV to −4.7 eV and the luminescent layer has an ionization potential φH of about −5.4 eV to −5.8 eV, there is caused a potential barrier ΔφA of as large as about −0.7 eV to −1.3 eV between the anode and the luminescent layer. Larger potential barriers make it necessary to increase application voltages between the anode and cathode to facilitate injection of positive holes from the anode into the luminescent layer so as to obtain an intended emission brightness, thereby making it difficult to drive organic EL elements at lower voltages. It is also difficult to attain a balance between positive holes and electrons to be injected from a cathode and an anode, respectively, thereby problematically failing to maintain stabilized light emission. Thus, the following three methods have been attempted.
The first method is to fix an ionization potential φ2 of an anode (transparent electrode: ITO), and to insert, between the anode and a luminescent layer, a buffer layer having an ionization potential at a level between those of ionization potentials φ2, φH of the anode and luminescent layer, respectively.
The second method is to fix an ionization potential φ2 of an anode (transparent electrode: ITO), and to select a luminescent layer having an ionization potential φH close to the ionization potential φ2 of the anode.
Contrary to the second method, the third method is to fix an ionization potential φH of a luminescent layer, and to select an anode having an ionization potential φ2 close to the ionization potential φH of the luminescent layer.